This invention relates to cold-storage appliances such as refrigerators and freezers for storing foodstuffs and other perishables. Other applications of the invention include storage of chemicals and medical or biological specimens.
The invention can be applied equally well to storing any items within a cooled environment, such as in a refrigerated goods vehicle. The term ‘appliance’ is therefore to be construed broadly, extending beyond fixed domestic devices into industrial, scientific and mobile applications. Nevertheless, the majority of this specification will describe domestic cold-storage appliances for storing foodstuffs.
The principal reason for storing foodstuffs in refrigerated conditions is to retard their degradation through microbial action or physiological or chemical changes, so that their shelf-life is extended for as long as possible. In order optimally to extend shelf-life through refrigeration, several factors must be considered; for example, the most suitable temperature for storing the foodstuff. In addition, some foodstuffs degrade quickly under the attack of virulent microbes that are readily circulated to other foodstuffs stored nearby, causing cross-contamination. It has long been appreciated that it is desirable to segregate different types of foodstuffs; accordingly, modem refrigerators are usually compartmentalised with the intention that the user will store similar types of food in each compartment.
As cold-storage appliances consume large amounts of power in use, energy efficiency is also an important consideration when designing such appliances. In fact, this is becoming an increasingly important consideration for consumers when they buy electrical appliances such as refrigerators, freezers, washing machines and tumble driers: retailers have responded to this by displaying energy-efficiency ratings on the front of such appliances in their showrooms. Indeed, this an EU requirement.
The typical household refrigerator is an upright cabinet with a hinged door on its front. The door and cabinet form an airtight enclosure via a magnetic compressible seal. Substantially all of the interior of the cabinet defines a storage volume, most commonly partitioned by a number of shelves. Access to all of the shelves is gained by opening the door, which is common to all shelves. The refrigerator also houses a cooler unit situated near to its top which cools the air circulating as a convection current in which the air cooled by the cooler unit sinks to the bottom of the refrigerator around the sides of the interior storage space, and as the air is warmed, it rises up through the centre of the interior back to the cooler unit where it is cooled, and so on ad infinitum.
To allow convection of the cool air around all of the refrigerator to ensure that all stored foodstuffs are chilled, the shelves are typically made of wire so that they offer little resistance to the circulation of air, whilst still being able to support the stored foodstuffs.
Upright freezers often follow this same basic design, although the cooler unit is used to chill the interior of the freezer to below zero Celsius. Upright refrigerators and freezers are often combined and sold as a single unit with a refrigerator occupying the top half of the cabinet and the freezer occupying the bottom half, or vice versa. As different temperatures are required for the two sections, they are partitioned by a solid divide and each section has its own door and cooler unit.
A major problem with upright refrigerators and freezers is the upright door. The coldest air will drop to the bottom of the refrigerator or freezer as it is more dense than warmer air. When the door is opened, this cold air flows freely out of the refrigerator or freezer to be replaced by warm ambient air flowing in at the top. Accordingly, whenever the door is opened, the rush of ambient air into the interior of the refrigerator or freezer causes its temperature to rise: this rise must be redressed by running the cooler unit and hence consuming more energy. Moisture in the incoming ambient air also gives rise to condensation and ice within the cabinet.
A further problem inherent in the upright door is its associated vertical seal. The coldest air that collects at the bottom of the refrigerator or freezer constantly seeks to escape through the sealing interface so that, if the seal forms an imperfect seal between the door and the cabinet, this air will escape.
A common alternative design of freezer is the chest freezer which, as its name implies, is an open-topped chest with a hinged lid closing its top. The interior of a chest freezer is usually partitioned using open wire walls or open wire baskets. As before, the walls or baskets are designed to allow cooled air to circulate freely throughout the freezer by convection.
The chest freezer helps to combat the problems of the upright refrigerator or freezer because its lid seal is not exposed to the coldest and densest air in the same manner. However, the chest configuration is inconvenient and wasteful of space because it precludes use of the space immediately above the freezer, which space must be preserved to allow the lid to be opened. Nor can items be left conveniently on top of the lid. It is also well known that large chest freezers can make access to their contents extremely difficult, it being necessary to stoop down and shift numerous heavy and painfully cold items to get to items at the bottom of the freezer compartment.
Moreover, both upright refrigerators and freezers and chest freezers suffer from a common disadvantage as follows. Generally, users want to access only one part of a refrigerator or freezer at a time. However, as there is generally only a single common door or lid for either a refrigerator or a freezer compartment, each time the door is opened the whole of its interior is exposed to warm ambient air, and so the whole of the interior must be cooled afresh at the expense of further energy consumption.
As mentioned above, segregation of different types of foodstuff is advantageous in avoiding cross-contamination. However, segregation of food is often compromised by the convection principle employed in most refrigerators. As the cooled air must chill the stored foodstuffs, it is circulated throughout the refrigerator. The substantially open baskets or shelves designed to promote circulation of air between the compartments unfortunately also promote the circulation of moisture and harmful bacteria. In addition, any liquid that may spill or leak from a foodstuff container will not be contained by the open partitions: this is particularly a problem for juices running from uncooked meats where the chances of contamination are high and the consequences of cross-contamination can be particularly severe.
As will be appreciated from the foregoing discussion, it is advantageous to divide a refrigerator into compartments, each with its own dedicated door or lid. Embodiments of this idea are disclosed in UK Patent Numbers GB 602,590, GB 581,121 and GB 579,071, all to Earle, that describe cabinet-like refrigerators. The front of the cabinet is provided with a plurality of rectangular openings for receiving drawers. Each drawer has a front panel larger than its respective opening so that a vertical seal is formed around the overlap when the drawer is in a closed position.
The drawers and their contents are cooled by a cooler unit that circulates cooled air by convection within the cabinet, in common with the types of refrigerator already described. To promote circulation of this air amongst all of the drawers, the drawers are open-topped and have apertures in their bottoms. Also, the drawers are disposed in a stepped arrangement, those at the top of the refrigerator extending back less far into the cabinet than the lower drawers so that the rear of each drawer is exposed to the downward flow of cooled air from the cooler unit.
Although only one drawer need be opened at a time, the apertures in the bottom allow cold air to flow freely from the open drawer, which is replaced by warm moist ambient air to the detriment of energy efficiency and with the increased possibility of crosscontamination. Indeed, when a drawer is opened, cold air within the cabinet above the level of that drawer will flood out, drawing ambient air into the cabinet. Furthermore, the drawers encourage ambient air to flow into the interior of the refrigerator because, upon opening, they act as pistons drawing the ambient air into the interior of the refrigerator cabinet. Once in the cabinet, the warm air can circulate as freely as the cold air that is supposed to be there.
Even when closed, the accumulation of cold air towards the bottom of the cabinet will exert increased pressure on the vertical seals of the lowest drawers, increasing the likelihood of leakage if the seal is faulty.
A further example of the above type of refrigerator is disclosed in UK Patent Number GB 602,329, also to Earle. The refrigerator disclosed therein suffers many of the above problems but is of greater interest in that a single drawer consisting of insulated sides and base is provided within the cooled interior of the cabinet. In contrast to the variants outlined above, the sides and base are solid and not perforated so that air cannot flow through them. When the drawer is closed, a horizontal member within the cabinet combines with the drawer to define a compartment, the horizontal member thus being a lid for the drawer. This compartment is provided with its own cooling coils situated just below the horizontal member.
Very little detail is given about the seal that is formed between the drawer and the horizontal member, other than that the horizontal member has a downwardly projecting rear end with a biased edge that makes a close fit with the rear wall of the drawer. Nothing else is said about the junction between the drawer and the horizontal member, apart from the general statement that the drawer is adapted when in its closed position to fit ‘fairly snugly’ against the horizontal member. It can only be inferred that the drawer and the horizontal member merely abut against each other. Whilst this will impede the passage of air into and out of the drawer, it will not form an impervious seal. As this is not a vapour seal, icing is likely to occur even when the drawer is closed.
The drawer arrangement described creates a compartment in which a different temperature can be set when compared to the essentially common temperature of the rest of the refrigerator. It is particularly envisaged that the drawer can act as a freezer compartment. The Applicant has appreciated a disadvantage in this arrangement, namely that as the freezer drawer resides within the cooled interior when closed, the outer surfaces of the drawer within the cabinet will be cooled to the temperature of the refrigerator. Accordingly, when the drawer is opened, those cooled outer surfaces will be exposed to ambient air containing moisture that will condense on the cooled surfaces leading to an undesirable accumulation of moisture. Condensation involves transfer of latent heat from water vapour to the drawer, thus increasing the burden of cooling the drawer again when the drawer is returned to the closed position within the cabinet.
Additionally, condensed moisture will be transferred to the interior of the refrigerator when the drawer is closed. As discussed above, the presence of water promotes microbial activity. A further disadvantage of introducing water into the interior of the refrigerator is that it may freeze: this can be a particular problem where the drawer of the enclosed compartment meets the insulated top, as any ice formation will form a seal that locks the drawer in a permanently closed position. This disadvantage was appreciated by Earle, as a cam mechanism is included in his proposal to break any ice-formed seals. It is also possible for a build-up of ice to affect the sealing ability of the seal, by preventing mating sealing surfaces from mating correctly.
Of course, the accumulation of ice on moving parts of the drawer mechanism is also undesirable as it will impede movement of the drawer.